PROJECT 2: PROJECT SUMMARY/ABSTRACT In the United States, castration-resistant prostate cancer (CRPC), is the second leading cause of male cancer- related deaths . Although there are new and highly potent androgen receptor (AR) targeted therapies for the treatment of metastatic CRPC (e.g. enzalutamide), tumor resistance develops in most patients. We have recently shown that under the selective pressure of systemic AR blockade, tumor glucocorticoid receptor (GR) expression increases and GR activity sustains pro-cell survival gene expression thereby enabling CRPC progression. The role of GR activity in prostate cancer (PC) is therefore dynamic and complex; initially when AR signaling is intact, GR activation appears to be mainly anti-proliferative and only later, in the context of sustained AR antagonism, does GR signaling contribute to tumor progression. The goal of this project is to determine how GR?s pro-cell survival activity develops over time as a function of AR activation state, and whether novel GR antagonists can mitigate GR activity thereby decreasing enzalutamide-resistance (Enza-R). Our central hypothesis is that following therapeutic AR blockade, the GR transcriptome shifts from predominantly anti-proliferative gene expression to one promoting cell survival. We therefore predict that an increasing percentage of Enza-R CRPC tumor cells will demonstrate significant GR expression that will in turn, undergo cytotoxicity with the addition ofr GR antagonism. Three aims are proposed to address this hypothesis. In Aim 1, we will analyze GR transcriptional function in PC over time before, during and after AR blockade. In Aim 2, we will investigate intratumoral heterogeneity of GR expression to test the hypothesis that focal (clonal) areas of high GR expression evolve exhibit GR-activation and enzalutamide resistance. Aim 3 is a translational aim in which we will investigate a novel SGRM in a phase I clinical trial in combination with enzalutamide as a treatment strategy for Enza-R CRPC. In addition, patient samples will be collected in the context of this trial to characterize AR and GR expression within circulating tumor cells (CTCs) from patients before and after treatment with enzalutamide and the SGRM. This work will help clarify the mechanisms by which GR activation contributes to PC progression and will inform strategies for blockade of GR-mediated pro-tumorigenic signaling pathways in patients.